1. Field of the Invention
The present invention relates to storage tanks for flammable liquids, and particularly to a vapor barrier for flammable liquid storage tanks that provides a vapor impermeable barrier layer with fire-suppressing capabilities for covering a surface of the flammable liquid.
2. Description of the Related Art
Flammable liquids, such as oil, gasoline and the like, must be stored in specialized storage tanks due to the flammable vapor that forms above the liquid surface. A common storage tank, often used in the petrochemical industry, is the “floating roof” tank. A typical floating roof tank is illustrated in FIG. 2. Tank 100 includes a hollow cylindrical housing 112 having an open upper end. The open upper end is sealed by a buoyant cover 114, having a circular contour matching the dimensions of the interior of housing 112. Cover 114 floats on the flammable liquid L contained within the housing 112, thus providing a seal between the surface of the liquid L and the outside environment, preventing the buildup of flammable vapor (and exposure thereof to external hazards, such as sparks).
Typically, the cover 114 is fabricated from metal and has a hollow chamber divided by walls into an array of pontoons in order to provide sufficient flotation to carry the weight of the cover plus additional weight, such as the weight of snow which might form on the cover 114. In older oil tank equipment, the cover was constructed of a metal plate with pontoons mounted beneath the cover plate, while modern tanks typically have the pontoons located above the metal cover plate. Repairs to the cover may require welding equipment, which can be used only after the tank has been taken out of service in order to ensure that the cover is clean and that there are no flammable vapors present. If any flammable vapors are present during repair work on the cover, such as the repair of a pontoon of the cover, a spark from the welding may ignite an explosive burning of the vapor.
Repairs may also be made without taking the tank out of service. For example, one of the pontoons may sustain a relatively small opening through which liquid can seep resulting in a loss of buoyancy. By means of an access port, a person may enter the pontoon and apply foamed urethane plastic as a liquid that later hardens to maintain buoyancy. Use of the plastic is not intended as a permanent repair because the plastic may become impregnated with the flammable liquid. Further, the plastic is disadvantageous because, at the conclusion of the service interval when reconditioning is mandatory, it is very difficult to remove the plastic so as to be able to clean the cover and make any permanent repairs. Obviously, welding cannot be employed for repair until all liquid and liquid soaked flotation, such as the foamed plastic, has been removed.
As an alternative procedure of repair, one might consider insertion in the pontoons of hollow, non-foamed plastic bodies to provide sufficient buoyancy so that it is not necessary to repair the leak in the pontoon. However, the use of a plastic hollow body, such as a hollow ball, has been avoided in the petrochemical industry because such a plastic body is electrically insulating and susceptible to developing a static electric charge. There is a danger that the flotation body may suddenly discharge via a spark, which can ignite an explosion.
Additionally, in the past, foam products have also been applied to the surfaces of flammable liquids, creating an effective vapor seal between the flammable liquid and the vapor space thereabove. However, the foam degrades within a short period of time, thus defeating the desired suppression qualities. Moreover, foam applied in the event of a flammable-liquids fire is the traditional form of fire fighting, with the intent of the foam being to cool the surface of the liquid and to also separate the flammable liquid from contact with oxygen, thus suppressing the fire. The difficulty with this traditional method of using foam is that the strong convective hot air currents caused by the fire tend to displace the foam, thus exposing the flammable liquid to the existing fire.
Further, marine vessels currently do not typically employ any physical barrier between a stored flammable liquid and the vapor space formed thereabove. Typically, such vessels employ inert gas generators that create an oxygen-deficient gas that is maintained above the flammable liquid in order to preclude the flammable vapor from mixing with oxygen that might otherwise create a flammable atmosphere. Such systems, however, do not provide backup prevention in case the gas generator fails.
Thus, a vapor barrier for flammable liquid storage tanks solving the aforementioned problems is desired.